Apache Reference Manual (4)

Syntax: ...
Context: server config, virtual host
Status: Core.
Compatibility: Available in Apache 1.3 and later
and are used to enclose a group of directives which will apply only to the named directory and sub-directories of that directory, the same as . However, it takes as an argument a regular expression. For example:

would match directories in /www/ that consisted of three numbers.
See Also: for a description of how regular expressions are mixed in with normal s.
See also: How Directory, Location and Files sections work for an explanation of how these different sections are combined when a request is received
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DocumentRoot directive
Syntax: DocumentRoot directory-filename
Default: DocumentRoot /usr/local/apache/htdocs
Context: server config, virtual host
Status: core
This directive sets the directory from which httpd will serve files. Unless matched by a directive like Alias, the server appends the path from the requested URL to the document root to make the path to the document. Example:
DocumentRoot /usr/web
then an access to http://www.my.host.com/index.html refers to /usr/web/index.html.
There appears to be a bug in mod_dir which causes problems when the DocumentRoot has a trailing slash (i.e., "DocumentRoot /usr/web/") so please avoid that.
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ErrorDocument directive
Syntax: ErrorDocument error-code document
Context: server config, virtual host, directory, .htaccess
Status: core
Override: FileInfo
Compatibility: The directory and .htaccess contexts are only available in Apache 1.1 and later.